The demand for colour mobile displays that are thin, light weight, low power, but clear and bright in all ambient light conditions has been increasing due to the increasing popularity of mobile phones, personal digital assistants (PDAs), digital cameras and laptop computers. The fact that these devices are required to work in varied ambient conditions and need high battery power has raised interest in transflective colour liquid crystal displays, which use a backlight to illuminate the display, but can reduce power consumption by making use of the ambient light in bright conditions.
In prior art transflective displays of twisted and non-twisted modes like TN (twisted nematic) and ECB (electrically controlled birefringence) are disclosed, wherein each pixel is split into a reflective and a transmissive subpixel (see for example Kubo et al., IDW 1999, page 183-187; Baek et al., IDW 2000, page 41-44; Roosendaal et al., SID Digest 2003, page 78-81 and WO 03/019276 A2). The transmissive subpixel has transparent front and back electrodes whereas the reflective subpixel has a transmissive front electrode and a reflective back electrode, requiring a patterned electrode structure which is achieved for example by “hole-in-mirror” technology.
As the transmissive mode uses half-wave (λ/2) optical modulation (λ=wavelength of incident light) and the reflective mode uses quarter wave (λ/4) optical modulation it was suggested to use a different cell gap (or LC layer thickness) for the subpixels, so that the reflective subpixel has about half the cell gap of the transmissive subpixel.
In order to make the reflective sub-pixel work with the transmissive subpixel, an achromatic (or “wide-band”) quarter wave foil (AQWF) is required to produce circularly polarised light (an AQWF exhibits an optical retardation of λ/4 for a wide wavelength band preferably encompassing the entire visible spectrum, and is formed for example by combining a QWF with a half wave foil (HWF, having an optical retardation of λ/2)). The AQWF also covers the transmissive pixel, hence requiring that an equivalent AQWF is placed on the backlight side of the cell.
However, the use of circularly polarised light in the transmissive portion of the display has the disadvantageous side-effect that twisted LC modes are less efficient at converting circular polarised light to the opposite handedness, thus reducing the brightness of the display and making the 90° twisted mode less effective.
To address the problems with circularly polarised light in the transmissive portion of the transflective display, it was proposed to use a patterned QWF having a pattern of areas with QWF retardation covering the reflective subpixels and non-retarding areas covering the transmissive subpixels (WO 03/019276; Van der Zande et al., Proceedings of the SID 2003, page 194-197). This allows the reflective and transmissive subpixels to be optimised separately and hence allows the use of linearly polarised light in the transmissive portion.
It has also been proposed, for example in US 2004/0075791 A1, US 2004/0032552 A1 and US 2003/0160928 A1, to use transflective displays of the vertically aligned (VA) mode.
Displays of the VA mode typically comprise an LC medium with negative dielectric anisotropy Δ∈ that has homeotropic orientation in the undriven state and is switched into planar orientation when an electric field is applied. In multidomain VA (MVA) displays the LC cell is additionally divided into multiple, typically four, perpendicular domains where (in the field-on state) the LC director is oriented into different directions, thereby providing symmetrical viewing angle characteristics and good colour performance. Multiple domains in MVA-LCDs can be formed by various methods, e.g. using patterned alignment layers, specific cell surface structures or electrodes with slots or adding polymeric material to the LC medium.
A transflective VA-LCD also requires an AQWF (i.e. QWF plus HWF) to work both in the reflective and transmissive mode. However, the presence of the AQWF leads to a reduced viewing angle and increased chromaticity. As an AQWF is also needed at the backlight side to give good transmissive and reflective performance, another drawback are increased manufacturing costs because at least four films are required: a QWF and HWF at the viewer's side and a QWF and HWF at the backlight side.
Hence, there is still a need for transflective displays that do not have the drawbacks of prior art displays described above.
It was an aim of the present invention to provide a transflective display that does not have the above mentioned disadvantages, shows high contrast, good brightness and low colour shift over a large range of viewing angles and is easy to manufacture in a time- and cost-effective way. Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
The inventors of the present invention have found that these aims can be achieved by providing transflective VA-LCDs as claimed in the present invention, comprising a patterned QWF inside the LC cell that improves the display performance and in particular produces reduced chromaticity in the reflective mode. Furthermore, by appropriate selection of the arrangement and orientation of the optical components in the display the optical performance, especially chromaticity, contrast and brightness can be improved.
DEFINITION OF TERMS
The term ‘film’ includes rigid or flexible, self-supporting or free-standing films with mechanical stability, as well as coatings or layers on a supporting substrate or between two substrates.
The term ‘liquid crystal or mesogenic material’ or ‘liquid crystal or mesogenic compound’ means materials or compounds comprising one or more rod-shaped, board-shaped or disk-shaped mesogenic groups, i.e. groups with the ability to induce liquid crystal (LC) phase behaviour. LC compounds with rod-shaped or board-shaped groups are also known in the art as ‘calamitic’ liquid crystals. LC compounds with a disk-shaped group are also known in the art as ‘discotic’ liquid crystals. The compounds or materials comprising mesogenic groups do not necessarily have to exhibit an LC phase themselves. It is also possible that they show LC phase behaviour only in mixtures with other compounds, or when the mesogenic compounds or materials, or the mixtures thereof, are polymerised.
For the sake of simplicity, the term ‘liquid crystal material’ is used hereinafter for both mesogenic and LC materials.
Polymerisable compounds with one polymerisable group are also referred to as ‘monoreactive’ compounds, compounds with two polymerisable groups as ‘direactive’ compounds, and compounds with more than two polymerisable groups as ‘multireactive’ compounds. Compounds without a polymerisable group are also referred to as ‘non-reactive’ compounds.
The term ‘reactive mesogen’ (RM) means a polymerisable mesogenic or liquid crystal compound.
The term ‘director’ is known in prior art and means the preferred orientation direction of the long molecular axes (in case of calamitic compounds) or short molecular axis (in case of discotic compounds) of the mesogenic groups in an LC material.
In films comprising uniaxially positive birefringent LC material the optical axis is given by the director.
The term ‘homeotropic structure’ or ‘homeotropic orientation’ refers to a film wherein the optical axis is substantially perpendicular to the film plane.
The term ‘planar structure’ or ‘planar orientation’ refers to a film wherein the optical axis is substantially parallel to the film plane.
The term ‘A plate’ refers to an optical retarder utilizing a layer of uniaxially birefringent material with its extraordinary axis oriented parallel to the plane of the layer.
The term ‘C plate’ refers to an optical retarder utilizing a layer of uniaxially birefringent material with its extraordinary axis perpendicular to the plane of the layer.
In A- and C-plates comprising optically uniaxial birefringent liquid crystal material with uniform orientation, the optical axis of the film is given by the direction of the extraordinary axis.
An A plate or C plate comprising optically uniaxial birefringent material with positive birefringence is also referred to as ‘+A/C plate’ or ‘positive A/C plate’. An A plate or C plate comprising a film of optically uniaxial birefringent material with negative birefringence is also referred to as ‘−A/C plate’ or ‘negative A/C plate’.
“E-mode” refers to a twisted nematic liquid crystal display (TN-LCD) where the input polarisation direction is substantially parallel to the director of the LC molecules when entering the display cell, i.e. along the extraordinary (E) refractive index. “O-mode” refers to a TN-LCD where the input polarisation is substantially perpendicular to the director when entering the display cell, i.e. along the ordinary (O) refractive index.
Unless stated otherwise, the term “polarisation direction” of a linear polariser means the polariser extinction axis. In case of stretched plastic polariser films comprising e.g. dichroic iodine based dyes the extinction axis usually corresponds to the stretch direction.